The present invention relates to bipolar transistors, and more particularly, to bipolar transistors formed in monolithic integrated circuit chips which are subject to charge generating disturbances.
Monolithic integrated circuit structural features have been shrinking rapidly in size in recent years. Along with this shrinkage, electrical currents and electrical charge accumulations formed and manipulated in integrated circuits based on these features have also been diminishing in value. As a result, charge accumulation quantities generated by certain charge generating disturbances which, in integrated circuits with larger feature sizes, would not be a problem are now quantities which are sufficiently large to cause difficulties in smaller feature integrated circuits.
In voltage level state switching circuits, such as logic circuits or memory circuits, which are constructed using such smaller integrated circuit structures, these disturbance charges can be sufficient to cause switching from an existing logic state to another at point in the circuit where such charge is generated. For instance, in an npn bipolar transistor, holes collected in the base from the emitter-base and the collector-base semiconductor pn junction regions as a result of such a disturbance could accumulate sufficiently to forward bias the emitter-base junction giving rise to sufficient additional base current to switch such a transistor, otherwise in the "off" condition, into the "on" condition. Also, electrons from such a disturbance collected from such a disturbance in the collector from the collector-base and the collector-substrate semiconductor pn junction regions could be sufficient in number to reduce the voltage at the collector so far as to again cause such a transistor to appear to switch from the "off" condition to the "on" condition.
Typically, such disturbances are local to the region near where the disturbance is generated and are temporary; thus, such a disturbance is often termed a "single event upset". Also, though the disturbance cause may be temporary, the results of the disturbance may be stored and are subject to being propagated further in the system which may lead to longer term and more significant effects.
A typical source of such charge generating disturbances is particle radiation. Such particles impinging on a monolithic integrated circuit will have "interactions" with the semiconductor material lattice structure and electrons along its path through the integrated circuit semiconductor material. This will result in raising the energy of the electrons involved into the conduction band and leaving corresponding holes in the valance band.
Should such electron-hole pairs be generated sufficiently close to a semiconductor pn junction, the electrons and holes so situated are subject to being collected by the action of electric fields in the region resulting because of voltage supplied to such junction and because of diffusion toward such junction. The structure of transistor devices and monolithic integrated circuits, and the method of operating both such devices and the circuits using them generally is such that only reverse-biased pn semiconductor junctions need to be considered. The collection of holes in the base and the collection of electrons in the collector that could lead to npn bipolar transistor voltage state changes, as indicated above, could be the result of such an impingement.
A further phenomena accompanying such radiation particle impingement across regions separated by a reverse-biased semiconductor pn junction is the "funneling" effect. This term is used to describe the process which such an impinging particle serves to, in effect, bend the depletion region boundary to allow the depletion region to follow the path of the particle. Thereby, the depletion region is extended in an elongated fashion along such a path to form a "funnel" in the semiconductor material. Such a "funnel" across the semiconductor pn junctions in a bipolar transistor could result in collector-to-emitter "punch through" leading to a short circuit between the collector and emitter of such transistor.
In a typical npn bipolar transistor, there is a substantially doped active base region provided directly between the emitter region and the collector region of the transistor. Further, there is a more highly doped inactive base region extending laterally from the active base region and continuing to the major surface of the integrated circuit semiconductor material to permit electrical contact to be made to the base region. Because the volume of the active base region is usually quite small, there is a relatively low number of electron-hole pairs generated by an impinging radiation particle in such region. Further, if one does so impinge, the lifetime of the generated electrons and holes will be small since recombination will occur rapidly in the highly doped inactive and active base regions such that relatively little current flow will result in the base region.
Such high doping also keeps the length of any "funnel" extending into the base region quite short, again because of rapid recombination. Further, the base region of a bipolar transistor is usually operated in such a manner that there is a relatively low circuit impedance in the external circuit between the base and emitter regions. This means that any currents that do form through the base region, because of charge generated in that region, are relatively unlikely to cause a significant voltage drop between the base region and the emitter region.
These circumstances lead to two of the above charge generation disturbance effects in bipolar transistors usually being relatively unimportant, that is, that the holes collected in the base region might switch a bipolar transistor from the "off" to the "on" condition, and that the possibility of a "funnel" occurring through the active base region might lead to a short circuit between the transistor emitter and collector regions.
The consequence of these effects being relatively unimportant is that the remaining effect, the collection of electrons in the collector region from the collector-base and the collector-substrate semiconductor pn junctions, is the dominant effect resulting from a radiation particle impingement in an npn bipolar transistor. This effect, which could lead to reducing the value of voltage occurring at the collector region sufficiently such that the collector would go from a high voltage state in a logic circuit to a low voltage state, is dominant because (i) the relatively large collector-substrate area, and because (ii) such npn transistors are usually constructed on a lightly doped p-type conductivity substrate.
Thus, a desirable npn bipolar transistor would have a construction which would minimize the accumulation of electrons in the transistor collector regions as a result of an impinging radiation particle, and the effects of any such accumulation. A further desire for such a construction is that it be compatible with structures useful in shrinking the size of such a transistor to thereby increase component density in monolithic integrated circuit chips using these transistors.